The present invention relates in general to thermocouple devices for flame detecting. Specifically, the present invention provides a thermocouple device particularly useful in situations requiring a high degree of corrosion resistance.
In certain industrial applications, it is necessary to sense the existence of a flame and automatically control a device in response to that sensing. For example, in a gas water boiler, it is desirable to sense a flame and maintain a flow of gas to a burner as long as the flame exists. Should the flame be extinguished, it is desireable to automatically control a valve to stop the gas flow.
One known device for flame sensing in such applications is the so-called flame-sensing rod. A flame-sensing rod is generally made of metal which is held near the flame. When a flame exists, an electric current flows between the flame-sensing rod and the burner. This current can be sensed to determine the existence of the flame. However a problem with using flame-sensing rods is that the current generated is quite small. In fact, it is too small to directly actuate any control device. The current must be sensed by a sensitive instrument and amplified.
An alternative device for flame sensing is the thermocouple device. A thermocouple device generally includes inner and outer thermocouple elements. The inner thermocouple element has a bar-shaped body that is made of a material, e.g. constantan, having a large thermoelectromotive force. The outer thermocouple element is formed as a cylindrical hollow body, which is made of a material, e.g. Inconel (a trade mark registered by the International Nickel Company), which is corrosion resistant, which surrounds the inner thermocouple element so as to prevent the inner thermocouple element from being corroded such as for example by water that may splash on the thermocouple device.
In conventional thermocouple devices, end portions of the thermocouple elements are joined by means of TIG (tungsten inert gas) welding. The end welded portion provides a detecting point which produces a large thermoelectromotive force when subjected to a flame. This thermocouple device has sufficient electric potential to directly drive a control circuit e.g. safety circuit. However, there is a drawback in this type of the thermocouple device. That is, the detecting point of the thermocouple device produced by welding is an alloy layer, including each metal of the thermocouple elements. The alloy has a corrosion resistance that is inferior to that of either of the individual metals of the thermocouple elements. When this thermocouple device is used in an environment where it may become wet, such as for detecting the flame of a gas-water boiler, the detecting point of the thermocouple device is apt to corrode. As a result of corrosion, the thermoelectromotive force is reduced. Therefore, there is a need to provide a flame sensor that not only provides a high thermal electromotive force, but also is corrosion resistant.